Local Softening Research Articles

Joule debonding of carbon reinforced polymer (CFRP) lap shear joints bonded with graphene nanoplatelets (GNPs)/epoxy nanocomposites

The potential of Joule heating CFRPs joints bonded with conductive graphene/epoxy nanocomposites as adhesives for a selective debonding was investigated. To ensure a localized softening of the bondline without altering the adherend’s structure, the epoxy used in the adhesive’s formulation was chosen to have a considerably lower Tg than the adherend. Joule heating the bondline considerably reduced the lap shear strength (LSS) relative to when the test was performed at room temperature, due to thermally induced structural changes promoted in the polymer network, which was consistent with the nanocomposites’ thermomechanical behavior predicted by DMTA. The minimum LSS value was reached in the vicinity of the adhesive’s Tg, allowing an ease deconstruction of the joints. SEM characterization of their fracture surfaces revealed that by controlling the adhesive’s formulation and their Joule heating the joints’ failure mechanism can be tuned to ensure the recovery of undamaged adherends that can be reused.

Tailoring microstructural stability and stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy via niobium addition

The influence of niobium (Nb) addition on the microstructural stability and 750 °C/135 MPa stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy designed for fourth-generation (Gen IV) nuclear reactors was systematically studied. The results showed that the stress rupture life first increased and then decreased with the increase of Nb content. The alloy with the addition of 0.5 wt% Nb exhibited the best stress rupture properties, with a stress rupture life of 164 h. Compared to Nb-free alloy, the stress rupture life has been increased by 382 %. The enhanced creep performance of this alloy was closely related to its excellent high-temperature microstructural stability, which can be explained as follows: homogeneous and optimal grain size distribution, elemental Nb provided good solid-solution strengthening effects, fine granular secondary carbides relieved stress concentration and strengthened grain boundaries (GBs), dynamic precipitation of nano-sized NbC carbides effectively pined dislocations. Dynamic recrystallization (DRX) behavior can be observed in all alloys. The mechanisms related to DRX behavior were discussed, and it was deemed that the smaller average grain size, higher blocky NbC volume fraction, and increased stacking-fault energy (SFE) may be responsible for the facilitated DRX in the alloy with higher Nb content, which greatly caused local softening and consequently, decreasing the creep resistance of the alloy. Important guidance for the further design and development of γ′-free Fe-Ni-based superalloys for Gen IV nuclear reactors with good high-temperature microstructural stability and creep performance could be summarized from the present study.

Equiaxed microstructure design enables strength-ductility synergy in the eutectic high-entropy alloy

Eutectic high-entropy alloys (EHEAs) represent attractive candidate materials for overcoming the strength-ductility trade-off, which can be enhanced through the directional alignment of the lamellar structure along the loading direction. Here, we put forward a new route to optimize the strength-ductility synergy without orientation dependence. Through a combination of severe plastic deformation and annealing, we convert the initially lamellar structure into a dual-phase structure comprised of ultrafine equiaxed grains. The significant grain refinement improves the yield strength from 703 MPa to 1199 MPa without sacrificing any ductility. During deformation, the localized softening resistance of the achieved dual-phase microstructure avoids necking, and the intrinsic microcrack-arresting mechanism effectively improves the fracture resistance. Grain boundaries and phase boundaries provide nucleation sites for dislocations and restrict dislocation transfer while the strain incompatibility is accommodated by geometrically necessary dislocations. This work demonstrates that dual-phase alloys comprised of ultrafine equiaxed grains provide a pathway for strengthening without loss of ductility.

Influence of ultrafine-grained structure of Ti-42Nb-7Zr alloy on energy dissipation and storage under quasi-static tension

The ultrafine-grained (UFG) structure and plastic deformation of the Ti-42Nb-7Zr alloy have been investigated using infrared thermography. The initial stage occurs at a constant temperature when the total energy of plastic strain absorbed by the alloy is 1.75 times higher for the UFG alloy, than for the alloy with a coarse-grained structure. This fact is due to substructural strengthening caused by severe plastic deformation and dispersion hardening with ω-phase nanoparticles. The amount of heat released before the UFG alloy fracture is 2 times larger than the amount of stored energy, and this results in localized softening of the UFG alloy prior to its fracture.

Two-stage work-hardening of a transformable B2-enhanced metallic glass composite by molecular dynamics simulation

The crystalline-amorphous interfaces play vital roles in affecting martensitic transformation, shear band nucleation and interface stability. Though both quasi-static and dynamic mechanical behaviors of shape memory enhanced bulk metallic glass composites have been studied via experiments, the atomic-level interactions among martensitic transformation, localized shear softening and interfacial strain concentration imposed by strain rate remain elusive. We employ molecular dynamics simulations to study strain rate effect on uniaxial compression behavior of transformable B2–CuZr enhanced bulk metallic glass composite. As strain rate increases, the proportion of martensitic transformation accelerates. During the competition among martensitic transformation induced-hardening, shear induced-softening and interface debonding, a two-stage work-hardening is observed, which is in agreement with experimental findings.

Portevin-Le Chatelier behavior in AlMgScZr alloys: Effects of Al3(Sc,Zr) dispersoid distribution and grain structure

Plastic instability caused by the Portevin-Le Chatelier (PLC) effect remains a challenge for Al–Mg alloy sheets during shape-forming processes, as it causes unsightly stretcher-strain markings on the surface of final products. In this study, the PLC behavior in annealed AlMg and AlMgScZr alloy sheets with varying Al3(Sc,Zr) dispersoid distributions and grain structures were investigated by combining tensile tests, microstructural characterization, and DIC analysis. The results show that the presence of Al3(Sc,Zr) dispersoids can inhibit the PLC effect, which is manifested by a decrease in serration amplitude, PLC band velocity, and an increase in critical strain. The inhibition effect is shown to be highly related to the distribution of Al3(Sc,Zr) dispersoids and resultant grain structures. The alloy with the densest dispersoid distribution and finest subgrain structure showed the slightest PLC effect. The reduction in serration amplitude and PLC band velocity is ascribed to the augmented resistance to local strain softening, caused by inhibition of dislocation motion by Al3(Sc,Zr) dispersoids and subgrain boundaries. In addition, a high density of Al3(Sc,Zr) dispersoids can strongly trap vacancies, which is responsible for the increase in critical strain.

Investigation of Two Laser Heat Treatment Strategies for Local Softening of a Sheet in Age-Hardening Aluminum Alloy by Means of Physical Simulation

AbstractCar manufacturers increasingly aid high-strength aluminum alloys for their advantageous weight-to-strength ratio, but their limited formability poses challenges in plastic deformation processes. Tailored heat-treated blanks (THTBs) are a propitious approach to improve formability. Surface laser treatment is the predominant technology for obtaining THTB. To design this process quickly and accurately, without material waste, the use of physical simulation is increasingly promising. It allows replicating the process on a lab-scale and studying posttreatment mechanical and metallurgical properties. By adopting Gleeble® physical simulator, this study investigates the softening effects of local surface laser heat treatment on a EN AW 6082 T6 aluminum alloy blank. Two laser movement strategies—single linear path and multiple rectangular paths—were investigated at two treatment speeds for each. A finite element (FE) model was developed for simulating the process under all explored conditions. FE-derived thermal cycles were reproduced by means of physical simulation. After physical tests, alloy mechanical properties were evaluated. Results show that these properties depend on both the peak temperature thermal cycle and the interaction time between the laser source and the material surface. The comparison between the two strategies revealed that the multiple rectangular paths strategy allows to achieve a wider softened area at comparable interaction times.

Spatio-temporal autogenous shrinkage and cracking behavior of core concrete in full-scale CFST: Insights from the world's largest span arch bridge

A full-scale concrete-filled steel tube (CFST) examination was performed on the Tian'e Longtan Bridge, featuring a 600-meter span and 0.9-meter steel tube diameter, to assess internal autogenous shrinkage distribution. This study employed vibrating wire strain gauges and distributed optical fibers to measure the shrinkage and detect cracking in the core concrete. Analysis of the test results led to the formulation of models that describe the axial and radial shrinkage behavior over time and space. Increased optical fiber strain highlighted regions undergoing local plastic softening and cracking. This research facilitates the early prediction and precise identification of potential cracking areas in the core concrete through optical fiber monitoring.

A biomechanical study of a polymer material bundled rib fracture fixator.

Rib fractures are one of the most common blunt injuries, accounting for approximately 10% of all trauma patients and 60% of thoracic injuries. Multiple rib fractures, especially flail chest, can cause local chest wall softening due to the loss of rib support, leading to paradoxical breathing, severe pain, and a high likelihood of accompanying lung contusions. This study investigates the mechanical properties of a new polymer material rib internal fixator to provide theoretical data for its clinical use. We conducted in vitro mechanical tests on 20 fresh caudal fin sheep ribs, using different fracture models across four randomly assigned groups (five ribs per group). The fixators were assessed using non-destructive three-point bending, torsion, and unilateral compression tests, with results averaged. Additionally, finite element analysis compared stress and strain in the polymer fixators and titanium alloy rib plates during bending and torsion tests. In vitro tests showed that the polymer fixators handled loads effectively up to a maximum without increase beyond a certain displacement. Bending and torsion tests via finite element analysis showed the polymer material sustained lower maximum equivalent stresses (84.455MPa and 14.426MPa) compared to titanium alloy plates (219.88MPa and 46.47MPa). The polymer rib fixator demonstrated sufficient strength for rib fracture fixation and was superior in stress management compared to titanium alloy plates in both bending and torsion tests, supporting its potential clinical application.

A maximum temperature rise model of the shear band in bulk metallic glasses

Based on mechanisms applicable to bulk metallic glasses (BMGs), such as a superposition principle of an instantaneous finite surface heat source temperature field and a flow model about the shear-band velocity, a maximum temperature rise model of shear bands in BMGs was deduced, ΔTmax=σycpΔupl·vSB2πα. The model describes the dependence of the maximum temperature rise on the yield strength and the shear-band velocity in BMGs. It can be calculated that the maximum temperature rise of BMGs is between 1 K ∼ 30 K at room temperature through these two parameters. A critical temperature, TC, of almost 0.8∼0.9Tg is introduced, at T<TC, the change of the yield strength with temperature in BMGs basically conforms a cooperative shear model (CSM), and at T>TC, a significant reduction of the yield strength in BMGs can be attributed to avoiding failure due to localized material softening.

Effect of cube texture on local softening of friction stir welded joints for nanostructured AA2024 processed by accumulative roll bonding

The current research provides an insight into the correlation between the crystallographic textures, microstructure, and hardness of friction stir welded joints in nanostructured AA2024 alloys processed through accumulative roll bonding (ARB). Utilizing varying rotational speeds (250, 500, 750, and 1000 rpm) at a constant traverse tool (150 mm/min) during friction stir welding (FSW), microstructural analyses reveal distinct grain structures and texture components in the nugget zone. The fully recrystallized Cube {001}⟨100⟩ texture-oriented grains appear at the rotational speed of 750 rpm. The hardness profiles of ARB-processed strips after FSW at different rotational speeds show local softening in the nugget zones. There might be a hypothesis concerning the dissolution of stable and metastable precipitates based on generated heat input, providing insights into the mechanisms influencing hardness variations. Notably, the examination of Cube {001}⟨100⟩ texture and its correlation with local softening adds a valuable dimension to the understanding of microstructural changes in FSW of nanostructured AA2024 alloys processed by accumulative roll bonding process.

Hydrogen enhanced localized plasticity in zirconium as observed by digital image correlation

Hydrogen embrittlement in zirconium alloys is a highly relevant research topic, important for ensuring the integrity of nuclear fuel clads. While the presence of hydrides in zirconium-based clad is a well-known risk factor due to their brittleness, the effect of the presence of hydrogen in solid solution is unclear.According to the Hydrogen Enhanced Localized Plasticity (HELP) model, hydrogen in solid solution may affect the mechanical performance of a metal by increasing dislocation mobility due to the shielding effect that hydrogen has on the interaction of dislocations with other types of defects. In the presented work, the presence of HELP has been assessed in Zircaloy-4 samples, under conditions relevant to the storage of spent nuclear fuel.Elevated temperature tensile tests were performed on samples containing localized areas of higher hydrogen concentration, as quantified by neutron radiography. Digital Image Correlation (DIC) analysis was performed on the specimen surfaces for quantitative characterization of strain heterogeneity during deformation to identify the regions where the plastic strain localization occurs.These regions could be correlated to elevated amounts of hydrogen in solid solution, that is, in the location immediately adjacent to dissolving hydrides, or in the areas of the samples with local hydrogen concentration close to the expected terminal solid solubility limit (TSS) in the test conditions.The results confirm the presence of localized softening caused by solid solution hydrogen, indicating that the HELP mechanism needs to be taken into account when modeling the mechanical response of zirconium-based nuclear fuel clads at elevated temperatures.

Modelling and experimental study of laser-assisted milling of fibre reinforced SiC/Ti-6Al-4V metal matrix composite

Metal matrix composites (MMCs) offer unique advantageous mechanical properties by strengthening a ductile metal matrix with a ceramic reinforcement (e.g., Ti6Al-4 V/SiCf). However, their heterogeneous composition poses machining challenges including fibre pullout, matrix cracking, and increased tool wear. Whilst pre-heating via laser-assisted machining (LAM) shows promise for improving machinability, traditional LAM implementations with a fixed laser spot size and straight laser path prevent uniform heating. By introducing spatially and temporally controlled LAM that generates homogeneous heating by varying the laser scanning velocity, a technique called fully inverse LAM can be applied for MMCs. This involves calculating separate temperature fields for the matrix and reinforcement to minimise thermal mismatch stresses. The fully inverse LAM decreases subsurface cracks and delamination resulting from conventional milling, whilst localised matrix softening reduces cutting forces by over 62 %. Flank tool wear is also diminished, increasing tool life by 120 %. Material analysis reveals reduced machined surface damage, lower surface roughness, and less formation of intermetallic compounds (Ti2C) compared to traditional LAM.

Mechanism of pin thread and flat features affecting material thermal flow behaviors and mixing in Al-Cu dissimilar friction stir welding

Three different features of pin models in Al-Cu dissimilar friction stir welding processes are established to analyse the effects of threads and flats on the two-phase material thermal flow behaviors. Based on computational fluid dynamics (CFD), a simulation methodology that calculates pins with complex morphologies through an adaptive force boundary is proposed. The pin characteristics are gradually optimized from “conical contour” to “conical-thread” and then to “thread-flats”. For dissimilar Al-Cu materials, the physical properties are weighted by the volume fractions of two-phase materials in the mixing zone. To build a dynamic adaptive contact model that varies with the material flow state, the interfacial friction shear stress is calculated by using the relative slip between the tool and material. The linkage effects between the “thread + flats” on the dissimilar Al/Cu welding mechanisms are systematically analysed. The enhancing mechanisms of the thread and flats on the material flow are different. The thread mainly reinforces the longitudinal migration of the Al/Cu materials, which is perpendicular to the horizontal plane. The flats effectively expand the plastic flow range and material mixing zone on the horizontal plane. These flow expanding effects are observed on the Z = 1 mm plane since the radial range of material flow enlarges from 2 mm to 3 mm under the action of the flats, while the horizontal flow range undergoes little change with the thread. The coupled mechanism of the “thread + flats” shows 0.06 s periodic strengthening, and the fluctuation threshold of the material flow velocity is wider, significantly improving the upwards stirring of the bottom material. The enhanced horizontal flow and vertical migration are beneficial to the uniform dispersion of the second-phase material (Cu) and local material softening for the formation of Al-Cu heterogeneous joints. Moreover, the accuracy of the model is verified by comparing the simulated and experimental TMAZ boundaries.

Wear factors and mechanisms of L-80 steel casings

Casing wear in directional drilling is inevitable and may result in catastrophic failure of the casing column. It is thus essential to understand its mechanisms and quantify its extent by estimating the casing wear factor. In this research, actual field casing samples, drilling pipe joints and muds were considered. An in-house built testing facility was used to test several L-80 casing samples by considering three rotational speeds of the drill pipe tool joint (DP-TJ) (115, 154, and 207 rpm) and three side loads (1000 N, 1200 N and 1400 N). The influence of the water- and oil-based muds on the coefficient of friction, wear volume, wear factor and wear mechanisms were investigated. The results revealed that under water-based mud (WBM) lubrication casing wear volume and wear factors were more than twice that of oil-based mud (OBM) lubrication. Moreover, it was observed that as the side load increased under both OBM and WBM lubrication at a constant rpm, both wear volume and wear factor increased. However, increasing the rotational speed while maintaining the side load constant decreased the wear factor, owing to the localized softening effect caused by the high heat generation at the contact area and the possible hydrodynamic lubrication regime change at higher speeds. The analysis of the digital microscopic images taken at the wear region shows that abrasive mechanism was dominant in the case of OBM lubrication. On the other hand, both abrasive and adhesive wear mechanisms were present under WBM lubrication.

Collapse mechanism of shield tunnel face and evaluation method for the minimum required support pressure considering soil strain softening effect

During shield tunneling, accurately revealing the collapse mechanism and evaluating the minimum required support pressure are key issues to avoid tunnel face failure. The strain softening effect of sandy soil is one of the vital factors for tunnel face stability analysis. However, it has not been explicitly considered in previous research. In this study, a practical strategy was proposed to consider the strain softening effect in finite element simulations. A novel method was also proposed for the evaluation of the minimum required support pressure. The results show that the stratum failure zone can be divided into three types during tunnel face collapse, which correlates with the cover to diameter ratio of the tunnel and can be uniformly delineated by the wedge-prim model. The process of tunnel face collapse was accompanied by the development of local stratum strain softening. The soil’s friction angle at the collapse zone boundary was found in a range between the peak value and the residual value. A coefficient representing the strain softening degree was defined to improve the traditional wedge-prim model. Moreover, by comparing with existing tests and theoretical models, the effectiveness of this method was well validated. The proposed method could accurately assess the minimum required support pressure for construction safety concerns.

Local fat content determines global and local stiffness in livers with simple steatosis.

Fat accumulation during liver steatosis precedes inflammation and fibrosis in fatty liver diseases, and is associated with disease progression. Despite a large body of evidence that liver mechanics play a major role in liver disease progression, the effect of fat accumulation by itself on liver mechanics remains unclear. Thus, we conducted ex vivo studies of liver mechanics in rodent models of simple steatosis to isolate and examine the mechanical effects of intrahepatic fat accumulation, and found that fat accumulation softens the liver. Using a novel adaptation of microindentation to permit association of local mechanics with microarchitectural features, we found evidence that the softening of fatty liver results from local softening of fatty regions rather than uniform softening of the liver. These results suggest that fat accumulation itself exerts a softening effect on liver tissue. This, along with the localized heterogeneity of softening within the liver, has implications in what mechanical mechanisms are involved in the progression of liver steatosis to more severe pathologies and disease. Finally, the ability to examine and associate local mechanics with microarchitectural features is potentially applicable to the study of the role of heterogeneous mechanical microenvironments in both other liver pathologies and other organ systems.

Shock compression and spall damage of dendritic high-entropy alloy CoCrFeNiCu

Plate impact experiments are conducted on a dendritic dual face-center cubic phase high-entropy alloy (HEA) CoCrFeNiCu, consisting of Cu-lean dendritic (DR) and Cu-rich interdendritic regions (ID). Free surface velocity histories are obtained along with microstructure characterizations. The Hugoniot equation of state and spall strength at different shock stresses are determined. Dislocation slip is the main deformation mode for CoCrFeNiCu HEA, and the dislocation density of the Cu-rich interdendritic regions increases more significantly during shock compression. Both ductile and brittle damage modes are observed. With increasing impact velocity, the Cu-rich interdendritic regions with severe strain localizations and more defects provide more damage nucleation sites. The spall strength increases firstly, reaches its maximum at a peak shock stress of 5.8 GPa, and then decreases. Molecular dynamics simulations reveal that the abnormal dependence of spall strength on peak shock stress is a result of strain localization and thermal softening in the Cu-rich regions.

A MODEL OF MICROSTRUCTURE EVOLUTIONUNDER HIGH- STRAIN RATE DEFORMATION OF COPPER

A physical model of high-strain rate deformation of metallic materials has been developed to describe the evolution of microstructure during the process. The model is based on representations of the mechanisms of grain boundary sliding, strain-induced grain growth and dynamic fragmentation of coarse grains developed in the framework of the theory of nonequilibrium grain boundaries. These representations are modified for the description of high-speed flow and supplemented by a model of dynamic recrystallization. It is shown that during high-strain rate deformation of metallic materials, grain boundary sliding dominates in the material regions with a fine-grained submicron structure and the material deforms in the superplasticity mode. In regions of material with larger grain size, grain volume deformation dominates, accompanied by fragmentation and formation of fine-grained structure with submicron grain size. The transformation of the structure from fine-grained to coarse-grained occurs due to strain-induced grain growth. Transformation of structure from coarse grain to fine grain occurs due to dynamic fragmentation. The transition from grain boundary sliding to grain volume deformation occurs at grain size corresponding to the optimum value for the course of dynamic recrystallization. The cyclic change of deformation mechanisms (from grain boundary strain-induced to grain volume deformation) is caused by alternation of the processes of deformation-stimulated grain growth and dynamic fragmentation. The alternation of the above cycles of grain boundary and intragranular deformation in different regions of the material makes it possible to maintain a high strain rate and suppress the processes of both local hardening and local softening. The high strain rate is achieved at the expense of a high strain of the regions in which grain sliding dominates. In the regions dominated by intragranular deformation, the structure is “prepared” for superplastic flow.

High performance magnetostriction of Fe-Cu-Pd ferromagnetic strain glass:Local chemical ordering induced martensitic nanodomains morphology with low modulus

Large magnetostriction with low saturation magnetic field was recently reported in Fe-Pd ferromagnetic strain glass alloys. Such behaviour is highly desirable for miniaturized sensor and actuator applications. The origin of high performance magnetostriction in Fe-Pd ferromagnetic strain glass is suggested to stem from its martensitic nanodomains. However, the atomic formation mechanism underlying these nanodomains and their associated local crystal symmetry breaking have not been well revealed. Moreover, accumulating experimental results indicate that the nano-structure can lead to a small saturation field but cannot guarantee large magnetostriction. Consequently, the necessary conditions for low field triggered large magnetostriction of Fe-Pd ferromagnetic strain glass alloys still remain unclear. In this paper, the chemistry, structure and transformation properties of the Fe98-xCu2Pdx (x = 28.5 ∼ 31) system are systematically explored to discover the origin of the high performance magnetostriction of ferromagnetic strain glass alloys. We found that the structure of ferromagnetic strain glass consists of local chemical order. This leads to tetragonal martensitic nanodomains with the local crystal symmetry breaking. The local chemical order also induces strong local phonon softening and a low modulus during the strain glass transition. Further investigation shows that the combination of martensitic nanodomains and low modulus are the necessary conditions to achieve both large magnetostriction and a low saturation field, because they can result in small energy barriers for the rotation of nanodomains. These findings provide an effective guideline to design high performance magnetostrictive materials and devices.